Tag: networking

I’ve been making more use of Cloudflare as a DNS and proxy/DoS protection service. I tend to use it by default now when I set up any kind of public-facing web application that might attract non-trivial traffic, store sensitive information, or where performance is a factor.

When spinning up a new server on an infrastructure platform like Digital Ocean, it’s almost certain that the IP address assigned to it is going to immediately see traffic from bad actors using automated attempts to find a web-based exploit. Even if you put Cloudflare in front of a web service and lock things down, connection attempts directly to the IP address will bypass their proxy and get through.

That’s where this first script comes in handy. Instead of allowing all tcp network activity to ports 80 and 443 to get through, I want to only allow traffic from known Cloudflare IP addresses. We can do this using the Ubuntu ufw firewall and Cloudflare’s published IP address blocks.

GadElKareem shared a script that I’ve adapted a bit to make use of ufw’s application profiles:

#!/usr/bin/env bash

set -euo pipefail

# lock it
PIDFILE="/tmp/$(basename "${BASH_SOURCE[0]%.*}.pid")"
exec 200>${PIDFILE}
flock -n 200 || ( echo "${BASH_SOURCE[0]} script is already running. Aborting . ." && exit 1 )
PID=$$
echo ${PID} 1>&200

cd "$(dirname $(readlink -f "${BASH_SOURCE[0]}"))"
CUR_DIR="$(pwd)"

wget -q https://www.cloudflare.com/ips-v4 -O /tmp/cloudflare-ips-v4
wget -q https://www.cloudflare.com/ips-v6 -O /tmp/cloudflare-ips-v6

for cfip in `cat /tmp/cloudflare-ips-v4`; do /usr/sbin/ufw allow from $cfip to any app "Nginx Full" comment "cloudflare"; done
for cfip in `cat /tmp/cloudflare-ips-v6`; do /usr/sbin/ufw allow from $cfip to any app "Nginx Full" comment "cloudflare"; done

It basically downloads known Cloudflare IPv4 and IPv6 addresses and then adds ufw rules to allow traffic from those addresses. I run this on a cron job like so:

# Refresh cloudflare IPs in ufw
0 7 * * * /root/bin/cloudflare-only-web-ports-ufw.sh >/dev/null 2>&1

The final step is to remove any ufw rules that allow traffic through to ports 80 and 443 for any source IP. This kind of rule may or may not be in place as a part of your existing server configuration.

The end result is that any web connection attempts not from Cloudflare will not be allowed through.

A second challenge with Cloudflare is making sure anything you do with visitor IP addresses found in access logs is using the actual original visitor IP instead of the Cloudflare proxy IP. An example would be using fail2ban to block accesses from a host that has tried too many times to gain unauthorized access to a server.

Fortunately, Cloudflare passes through the original visitor IP as a header, so we just have to make use of that in our logs. This script, originally shared by ergin on GitHub, will download the published Cloudflare IP addresses and generate an nginx-friendly configuration file that adjusts the “real IP” of the visitor for logging and other purposes:

#!/bin/bash

CLOUDFLARE_FILE_PATH=/etc/nginx/conf.d/cloudflare-realips.conf

echo "#Cloudflare" > $CLOUDFLARE_FILE_PATH;
echo "" >> $CLOUDFLARE_FILE_PATH;

echo "# - IPv4" >> $CLOUDFLARE_FILE_PATH;
for i in `curl -s https://www.cloudflare.com/ips-v4/`; do
    echo "set_real_ip_from $i;" >> $CLOUDFLARE_FILE_PATH;
done

echo "" >> $CLOUDFLARE_FILE_PATH;
echo "# - IPv6" >> $CLOUDFLARE_FILE_PATH;
for i in `curl -s https://www.cloudflare.com/ips-v6/`; do
    echo "set_real_ip_from $i;" >> $CLOUDFLARE_FILE_PATH;
done

echo "" >> $CLOUDFLARE_FILE_PATH;
echo "real_ip_header CF-Connecting-IP;" >> $CLOUDFLARE_FILE_PATH;

#test configuration and reload nginx
nginx -t && systemctl reload nginx

I run this on cron like so:

# Refresh cloudflare IPs and reload nginx
30 4 * * * /root/bin/cloudflare-ip-whitelist-sync.sh >/dev/null 2>&1

Because the config file is output in the /conf.d/ directory in the nginx main directory, the default nginx config will pick up its contents without further action.

Continuing with the example of using fail2ban to block unwanted traffic from abusive hosts, you can set up a set of custom Cloudflare ban/unban actions in a file like /etc/fail2ban/action.d/cloudflare.conf, using this version adapted from the original by Mike Rushton:

[Definition]
actionstart =
actionstop =
actioncheck =

actionban = curl -s -X POST -H 'X-Auth-Email: <cfuser>' -H 'X-Auth-Key: <cftoken>' \
            -H 'Content-Type: application/json' -d '{ "mode": "block", "configuration": { "target": "ip", "value": "<ip>" } }' \
            https://api.cloudflare.com/client/v4/user/firewall/access_rules/rules

actionunban = curl -s -o /dev/null -X DELETE -H 'X-Auth-Email: <cfuser>' -H 'X-Auth-Key: <cftoken>' \
              https://api.cloudflare.com/client/v4/user/firewall/access_rules/rules/$(curl -s -X GET -H 'X-Auth-Email: <cfuser>' -H 'X-Auth-Key: <cftoken>' \
              'https://api.cloudflare.com/client/v4/user/firewall/access_rules/rules?mode=block&configuration_target=ip&configuration_value=<ip>&page=1&per_page=1' | tr -d '\n' | cut -d'"' -f6)

[Init]
cftoken =
cfuser =

and then in the jail.local file you can set this up as the default action:

cfemail=<your cloudflare@email here.com>
cfapikey=<your cloudflare API key here>

# Define CF ban action without mailing
action_cf = cloudflare[cfuser="%(cfemail)s", cftoken="%(cfapikey)s"]

# Set default action to Cloudflare ban/unban
action = %(action_cf)s

The end result is that bad actors never get past Cloudflare’s proxy protection to attempt additional foolishness directly on your server.

Thanks to all of the folks who created the original version of these scripts. If you have suggestions for improvement or your own fun Cloudflare tooling, please share!